The present invention relates to an image forming apparatus of the type having photoconductive means and transfer means which are each implemented with a drum and are driven independently of each other to be indiviually rotatable at variable speeds, and accelerating the rotation of the transfer drum relative to that of the photoconductive drum during the interval between consecutive image transfers in matching relation to a size of paper sheets so as to increase the copying speed, and a control system for such an apparatus. More particularly, the present invention is concerned with a color copier or like color image forming apparatus capable of reducing a period of time necessary for copying, or copying time, and a control system for such an apparatus.
In a prior art color copier, it has been customary to adopt an arrangement wherein a color original document is repetitively scanned by optics which includes a plurality of color separating filters while, at the same time, exposures by a plurality of separated color components are sequentially effected. The resulting latent images formed on a photoconductive drum, or photoconductive means, are individually developed by toner of complementary colors which are supplied by a developing device, and the resulting toner images are sequentially transferred to a paper sheet which is clamped on the transfer drum, or transfer means, which is in turn held in contact with the photoconductive drum. The photoconductive drum and the transfer drum are interconnected by gears or the like which involves little backlash so as to be driven together and each at a constant speed. The optics are driven by, for example, a servo motor which rapidly responds to speed control. A problem with this kind of driving system is that an extra gear train and other elements needed to operatively connect the photoconductive and transfer drums to each other increase the overall size of the apparatus. Another problem is that mechanical vibrations ascribable to the gears and other are apt to bring about jitter, failure of register, damage to images and other undesirable occurrences. In addition, such a number of structural elements have be to individually machined with substantial accuracy and result in difficult maintenance as well as in poor durability and reliability.
On the other hand, a prerequisite with a prior art color copier of the type described is that respective color components reproduced by consecutive transfers be accurately registered to provide a copy of high quality. This prerequisite cannot be satisfied unless the circumferential length of one of the photoconductive and transfer drum is an integral multiple of that of the other, as generally accepted. Specifically, assuming that the photoconductive drum has a circumferential length of P while the transfer drum has a circumferential length of T, they have to be designed such that an equation T=n.multidot.P (n=1, 2, 3 . . . ) holds when T is greater than P and an equation P=n.multidot.T holds (n=1, 2, 3 . . . ) when T is smaller than P. Otherwise the above-described kind of drum driving system which relies on gears or the like fails to drive the optics, photoconductive drum and transfer drum in synchronism and, especially, it prevents the consecutive color-by-color transfers in a color combining mode from being started at the same positions. For this reason, despite that the circumferential length of the transfer drum need only be slightly greater than the longitudinal dimension of format A4 of general purpose PPC paper sheets which are extensively used today, it has heretofore been dimensioned far greater than the same.
In the above-described driving system, the rotation of the transfer drum, for example, follows that of the photoconductive drum so that the copying time remains the same with no regard to the format of paper sheets. Therefore, it is impossible for the transfer drum to be accelerated relative to the photoconductive drum after the trailing edge of a paper sheet of comparatively small format has moved past a transfer position, for the purpose of speeding up the copying operation. A control system capable of setting up an adequate copying time which matches itself to a paper size is disclosed in Japanese Laid-Open Publication (Kokai) No. 60-218673. The system disclosed uses a scanning sensor responsive to a scan start position of the optics, and a paper sensor disposed near the transfer drum to sense the trailing edge of a paper sheet loaded on the drum. The times at which a transfer is started and ended are determined on the basis of the ouput signal of the scanning sensor and that of the paper sensor, respectively. During the interval between the times of the start and end of transfer determined so, the rotation speed of the transfer drum is variably controlled to register the leading edge of the paper sheet and that of each toner image representative of a particular color component.
Such a system, however, cannot be accomplished without increasing the cost because the paper sensor responsive to the trailing edge of a paper sheet has to be associated with the transfer drum. Further, the accuracy of detection attainable with the paper sensor is limited and, therefore, the entire system lacks reliability.